Synthesis of myosin isozymes will be analyzed in the perfused working heart under pressure overload conditions, or after administration of thyroxine. Such stimuli induce two types of cardiac hypertrophy, one in which myosin is increased in ATPase activity and the other where myosin has a decrease in ATPase activity. We will subject the animal to both a severe pressure overload and a mild overload. We will compare changes in myosin with the two types of overload with that obtained from thyroxine administration. We will assess whether one hypertrophying stimulus relative to the next, can increase the tissue PCO2 of the perfused heart and whether tissue acidity can then change the calcium binding characteristics of regulated actomyosin. We will prepare CNBr peptides from the highly labeled, newly synthesized myosin, and analyze such peptides on two-dimensional gel electrophoresis.